Hydraulic vehicle brake systems are known which are configured to be operable by external force and which include, in addition to a brake master cylinder actuable by muscle power to which wheel brakes are connected hydraulically and which supplies pressure and volume for actuating wheel brakes, a further, electrically controllable pressure and volume supply unit which activates the wheel brakes in a brake-by-wire operating mode. In the event of failure of the electrically controllable pressure and volume supply unit, the wheel brakes are actuated solely by the muscle power of the vehicle driver (unboosted fallback operating mode).
An electrohydraulic brake system comprising a brake master cylinder actuable by the brake pedal, a travel simulator and a pressure supply unit is known from WO 2011/029812 A1. In a brake-by-wire operating mode the wheel brakes are subjected to pressure by the pressure supply unit. In the fallback operating mode the wheel brakes are subjected to the pressure exerted by the driver by means of the brake master cylinder actuable by the brake pedal. It is considered disadvantageous in the known brake system that, in the event of failure of the activation or drive of the electrically controllable pressure supply unit, large brake pedal forces must be exerted by the driver in order to attain sufficient deceleration using the service brake.